Warming of the earth as a result of the current atmospheric increase in CO.sub.2 is predicted and the demand for cleaner energy sources is increasing. Nuclear power generation, although not involving CO.sub.2 atmospheric discharge, still has numerous unsolved problems. Thus, the development of a safer and cleaner energy source is demanded by the environmentally conscious public.
Of the various energy sources expected to be used in the future, solar cells are considered particularly promising because of they are clean, safe, easy to use and nondepletable.
Of the various types of solar cells, thin film amorphous silicon solar cells have demonstrated great potential for widespread future use because they can be manufactured by mass production techniques in large areas and at low cost. However, any large area solar cell will inherently include a large number of electrically short-circuited portions of the active region thereof, which short circuit portions could result in significantly reduced yields and low photovoltaic conversion efficiencies.
A thin-film solar cell formed on an electrically conductive substrate, such as a metallic substrate, has the advantage of being light-weight and shatter resistant, vis-a-vis, crystalline or polycrystalline solar cells. Typically, a thin stainless-steel web or a synthetic plastic resin with an electrically conductive layer formed thereon can be employed as the substrate for the thin film amorphous silicon solar cell. Such a solar cell can be produced by successively depositing layers of a lower electrode material, a semiconductor body, a transparent conductive material, etc. on the aforementioned substrate by the continuous roll-to-roll process set forth in U.S. Pat. No. 4,400,409, the disclosure of which is incorporated herein by reference.
However, the surface of a metal substrate, e.g., that of a stainless-steel substrate, is not perfectly smooth, but rather exhibits microscopic, morphological flaws or irregularities which extend above the deposition surface thereof. Such surface irregularities cannot be uniformly covered by the relatively thin body of semiconductor material (less than a micron in total thickness). Further, due to contaminants which are deposited during the thin-film CVD process, the deposition surface of the substrate may include pinholes, thereby causing a large number of current shunting portions (a portion in which an electric current follows a lower resistance pathway rather than being collected by the grid electrodes and/or bus bars of the solar cell) or short-circuiting portions to form between the upper and lower electrodes, resulting in the deterioration of the photoconversion efficiency of the solar cell. It is to be noted that for purposes of this disclosure, the terms "shunts" and "short circuits" will be used synonymously.
The foregoing problem can best be seen from a perusal of the drawings. FIG. 1 is a schematic cross-sectional view showing a short circuit portion of a thin film photovoltaic device 1 which has been formed on an electrically conductive substrate. In the drawing, the reference numeral 2 indicates the electrically conductive substrate, 4 indicates a layer of back-reflection material, 6 indicates a multilayered body of semiconductor material which includes a photogenerative region, 8 indicates a layer of transparent conductive material, 10 indicates an upper electrode for collecting photogenerated change carriers, 12 indicates an irregularity which shunts or short circuits said photogenerated change carriers, and 14 indicates an irregularity extending from the upper collecting electrode into and through the semiconductor body 6 so as to form a shunt or short circuit for photogenerated change carriers. The conductive substrate 2 also serves as the lower electrode of the solar cell. Generally, the shunt or short circuit portions 12 and 14 can be of two types: the type in which the lower electrode 2 and the upper collecting electrode 10 are short-circuited; and the type in which the short circuit path is caused when the upper collecting electrode 10 is formed on and in a subjacent pinhole section which exposes a portion of the lower electrode 2, i.e., the lower electrode is not covered with a layer of the thin film body of semiconductor material.
Accordingly, the electric current photogenerated in the semiconductor body 6 by the light incident on the surface of the transparent conductive layer 8 is at least partially diverted by the current shunting paths formed in the photovoltaic device 1, with the result that the photovoltaic-conversion efficiency deteriorates.
In order to solve the aforementioned problem, the present inventors have proposed various photovoltaic-device structures in U.S. Pat. Nos. 4,590,327, 4,633,033 and 4,633,034, the disclosure of which are incorporated herein by reference.
U.S. Pat. No. 4,590,327 discloses a photovoltaic device in which the current carrying section of the collecting electrode, composed of a finger-like grid assembly to collect photogenerated current and a bus-bar adapted to carry the current collected by the grid, is operatively disposed above the semiconductor body such that said the bus bar portion of the collecting electrode is insulated therefrom by a solid layer. If, in this structure, a pinhole exists below the finger-like grid assembly where no solid layer exists, through which any portion of the back-reflection layer or the conductive substrate is exposed, a shunt or short circuit condition results.
In the, photovoltaic structure disclosed in U.S. Pat. No. 4,633,033, the collecting electrode is operatively disposed between the transparent conductive layer and the semiconductor body. Directly below this collecting electrode, there is disposed a layer of high-resistance material for restricting the flow of short circuit-path current. If, in this structure, either (1) the grid portion of the collecting electrode, is formed by screen printing a conductive ink or paste or, (2) the high-resistance layer is formed from a resin ink or paste containing highly resistant organic macromolecules, gas is discharged during heating step, it becomes difficult to form a transparent electrode layer of satisfactory quality.
In the photovoltaic-device structure disclosed in U.S. Pat. No. 4,633,034, the upper collecting electrode is operatively disposed on the optically transparent conductive layer, and, a patterned layer of high electrical resistance material is disposed between that transparent conductive layer and the substrate. The patterned material layer is made about 10% larger than the overlying bus bar portion of the collecting electrode so as to restrict the flow of short circuit current. Like the photovoltaic structure disclosed in aforementioned U.S. Pat. No. 4,633,033, the '034 structure has the problem that, if the high-resistance layer is formed from a resin paste containing highly resistant organic macromolecules, gas is evolved during heating, thereby making it difficult to form high quality layers of semiconductor material and transparent conductive material.
U.S. Pat. No. 4,729,970 discloses a method in which, if a shunt or short circuit path should develop between the lower electrode and the upper electrode of transparent conductive material of a photovoltaic device, electrolysis is effected in an electrolyte disposed between the electrically conductive substrate or the lower electrode and the counter upper transparent electrode, thereby insulating the lower electrode from the short-circuited section of the upper transparent conductive electrode. However, when the collecting electrode is formed on either (1) an irregularity formed in a protruding section of the electrically conductive substrate and said protruding section extends through the thin film body of semiconductor material; or (2) a portion where a part of the lower electrode or a part of the conductive substrate is exposed through a pinhole in the body of semiconductor material and the layer of transparent conductive material, a shunt or short circuit path will still be generated.
Further, if the electrolytic process described in U.S. Pat. No. 4,729,970 is applied to a photovoltaic device having the structure depicted in U.S. Pat. No. 4,633,033, the collecting electrode or the layer of high-resistance material could delaminate. Likewise, if the electrolytic process of U.S. Pat. No. 4,729,970 is applied to a photovoltaic device having the structure depicted in U.S. Pat. No. 4,633,034, the collecting electrode, the layer of high-resistance material, or the body of semiconductor material could delaminate.